In a propulsion assembly in which the turbine engine is a turbojet, the external fairing device that surrounds the turbojet usually comprises an outer annular wall or envelope and an inner annular wall or envelope. The outer annular wall forms an external fairing along which the relative wind flows during flight, while the inner annular wall will guide an air flow supplying the turbine engine. In the case of a double flow turbojet, the air flow directed by the inner annular wall is the bypass flow propelled by the fan and that flows in the downstream direction around the turbojet core.
FIG. 1 is a partial axial sectional view of a propulsion assembly 10 of a known type of aircraft, comprising a double flow turbojet 12 and an external fairing device 14 surrounding this turbojet.
As shown in this figure, the outer annular wall 16 surrounds the propulsion assembly 10 so as to guide relative wind 18 around the propulsion assembly while the inner annular wall 20 directs the air flow 22 penetrating in the propulsion assembly at the upstream end, and the bypass flow stream 24 originating from the above mentioned air flow 22 and flowing around the core 26 of the turbojet 12 at the downstream end.
Throughout this description, the “upstream” and “downstream” directions are defined relative to the global gas flow direction in the turbine engine and the terms “forward” and “aft” should be considered relative to a direction of motion F of the aircraft under the effect of the thrust applied by the turbojet 12, this direction being parallel to a longitudinal axis 72 of the turbojet.
The outer 16 and inner 20 annular walls delimit an internal compartment 28 between them in the external fairing device 14 also called the “fan compartment” because of its position adjacent to the fan 30, between an air intake 32 and a thrust inverter 34.
In general, the outer wall 16, an upstream portion of the inner wall 20 corresponding to the air intake 32 and a downstream portion of the inner wall 20 corresponding to the thrust inverter 34, form part of a nacelle of the propulsion assembly 10. On the other hand, an intermediate portion of the inner wall 20 that delimits the internal compartment 28 forms part of an intermediate casing 35 fixed to the turbojet core 26.
The internal compartment 28 usually contains an accessory control box 36 also called the AGB (Accessory Gear Box) that mechanically connects a rotor 38 of the turbine engine to one or several accessories such as a starter. In the example shown, the rotor 38 concerned is a high pressure compressor rotor and is connected to the AGB 36 through an intermediate shaft 40 extending from the turbojet core 26 as far as the internal compartment 28.
Furthermore, the internal compartment 28 is usually used to house one or several systems such as an Engine Control Unit (ECU), although none of these systems are shown in the sectional plane in FIG. 1.
Some of these systems usually require cooling in order to operate satisfactorily.
Furthermore, safety standards impose that the concentration of inflammable vapours in the air inside the internal compartment 28 should not exceed a predefined maximum level, such that the internal compartment must be ventilated. “Ventilation” means that the air must be renewed regularly within the internal compartment 28.
The external fairing device 14 usually comprises an air inlet orifice 42 and an air exhaust orifice 43 formed in the outer wall 16, to ventilate the internal compartment 28 and to cool any systems contained in it. The two orifices 42 and 43 are usually diametrically opposite as shown in FIG. 1. Furthermore, the air inlet orifice 42 is usually arranged close to the upstream end of the internal compartment 28 while the exhaust orifice is typically arranged close to the downstream end of the internal compartment 28.
At the air inlet orifice 42, the outer wall 16 is usually profiled so as to form a dynamic air intake. To achieve this, the upstream edge 44 of the air inlet orifice 42 may for example be curved inwards into the internal compartment 28 as shown in FIG. 1 to form an air intake of the type typically referred to as “NACA”, to facilitate penetration of the air boundary layer circulating along the outer wall 16, in the air inlet orifice 42. As a variant or as a complement, the downstream edge of the air inlet orifice 42 may be convex outwards to form a scoop, which also facilitates sampling of air circulating along the outer wall 16. As a variant, such a scoop may be formed from an add-on part on the outer wall 16, the add-on part facing the air inlet orifice 42.
The air flow that enters the internal compartment 28 then forms a ventilation flow 46 circulating in the internal compartment and that finally goes out through the air exhaust orifice 43.
However, the cooling efficiency of the internal compartment 28 then depends on the speed of the relative wind 18 surrounding the external fairing device 14 and therefore the aircraft advance speed.
Thus, when the aircraft moves at low ground speed or is stopped, the above-mentioned dynamic air intake becomes inoperative and the internal compartment 28 of the external fairing device 14 is only cooled by natural convection through the air inlet orifice 42 and the air exhaust orifice 43, but this may be insufficient.
One solution for providing satisfactory ventilation and cooling whenever possible under these low speed conditions consists of oversizing the air inlet 42 and air exhaust 43 orifices, and/or exaggerating the aerodynamic profile of the outer wall 20 at the edge of the air inlet orifice 42, so as to increase the ventilation flow 46.
However, this solution penalises the aerodynamic drag of the propulsion assembly, particularly due to an increase in the ram drag during flight, and particularly during the cruise phase.
Furthermore, if a fire occurs in the internal compartment 28, the circulation of relatively large ventilation air flow 46 is not desirable because this increases the oxygen content in the air inside the internal compartment, which may make it more difficult to put the fire out. The external fairing devices are usually equipped with a fire fighting system designed to output an extinguishing agent within the internal compartment 28. The relatively large ventilation air flow then makes it necessary to oversize the fire fighting system so that it will be capable of maintaining a sufficient concentration of extinguishing agent inside the internal compartment 28 in case of fire.
In general, in the case of a fire, the ventilation air flow 46 in the internal compartment 28 cannot be interrupted or at least reduced, which tends to reduce the efficiency of the extinguishing agent.
Furthermore, in double flow turbojets, as shown in FIG. 1, the bypass flow 24 is separated from the turbojet core 26 by an internal fairing device 48 comprising an outer wall 50 and an inner wall 52 together delimiting an internal compartment 54, typically called the “core compartment”. The outer wall 50 is sometimes called the IFS (Inner Fixed Structure).
In order to limit the concentration of inflammable vapours, this internal compartment 54 also has to be ventilated, which is usually done by an air inlet orifice 56 usually on the upstream side and an air exhaust orifice 58 usually on the downstream side. These orifices 56 and 58 carry the circulation of a ventilation air flow 60 inside the internal compartment 54. The orifices 56 and 58 are preferably diametrically opposite each other.
In particular, the air exhaust orifice 58 is preferably arranged downstream from a downstream end 62 of the external fairing device 14 such that the air pressure circulating along the outer wall 50 of the internal fairing device 48 at the air exhaust orifice 58 is as low as possible. As a variant, the air exhaust orifice 58 may communicate with a channel passing through an arm connected to the external fairing device 14 and opening up on the outside of the propulsion assembly through the outer wall 16 of the external fairing device 14.
The air inlet orifice 56 may be in the form of a dynamic intake provided that the outer wall 50 is given an appropriate conformation, as explained above concerning the external fairing device 14.
Furthermore, circulation of the ventilation flow 60 in the internal compartment 54 also provides a way of cooling any systems housed within this compartment, if necessary.
However, the ventilation flow 60 passing through the internal compartment 54 of the internal fairing device 48 may be insufficient, particularly in turbojets with high dilution ratios.
However, the solution consisting of oversizing the air inlet 56 and air exhaust 58 orifices penalises the performances of the turbojet and also has the disadvantages described above concerning the risk of fire.
Similar problems arise concerning the fairing device surrounding the core of a turboprop or an open rotor turbojet.